On the Approximability by Finite $p$-Groups of Free Products of Groups with Normal Amalgamation

A sufficient condition for the residual $p$-finiteness (approximability by the class $\mathscr F_p$ of finite $p$-groups) of a free product $G=(A*B;H)$ of groups $A$ and $B$ with a normal amalgamated subgroup $H$ is obtained. This condition is used to prove that if $A$ and $B$ are extensions of residually $\mathscr N$-groups by $\mathscr F_p$-groups, where $\mathscr N$ stands for the class of finitely generated torsion-free nilpotent groups, and if $H$ is a normal $p'$-isolated polycyclic subgroup, then the group $G$ is residually $p$-finite (i.e., residually $\mathscr F_p$-group), provided the quotient group $G/H^pH'$ is residually $p$-finite.